Pulmonary arterial hypertension (PAH) is a progressive fatal disease. Current therapies focus mainly on vasodilators, which relieve symptoms and improve quality of life, but do not affect mortality. These drugs do not slow the progression of vascular remodeling (marked by muscularization and plexiform formation), which contributes to the characteristic high vascular resistance and compliance seen in PAH. There is an urgent need to elucidate novel molecular mechanisms of vascular remodeling in the context of PAH and develop new therapeutic targets. The pathogenesis of PAH includes endothelial cell (EC) apoptosis and inflammation, and vascular smooth muscle cell (VSMC) proliferation and migration. Our lab's previous studies show a pathogenic role for CypA in atherosclerosis and aneurysm formation - CypA exerts pro-inflammatory and pro-apoptotic effects on EC, and also stimulates SMC proliferation, SMC migration, and MMP activation in SMC. The pathogenesis of PAH is similar mechanistically, and as such, we propose CypA is a key mediator of PAH. Four novel findings drive this proposal. 1. CypA expression is significantly increased in plasma of PAH patients, and rats with severe PH (pneumonectomy followed by monocrotaline, P-MCT). 2. EC-specific CypA overexpressing transgenic mice (ecCypA-Tg mice) show elevated pulmonary artery pressure and remarkably increased expression of smooth muscle actin in lungs. 3. Acetylated (AcK)-CypA is significantly increased in ecCypA-Tg mice, P-MCT rats, and is present in PAH patients. AcK-CypA is more potent than CypA in stimulating ERK1/2 in lung VSMC, and apoptosis in EC suggesting a new drug target. 4. The relative importance of intracellular CypA vs. extracellular CypA (eCypA) has been debated. We show that MM284 (a cell impermeable CypA PPIase inhibitor), in a dose-dependent manner, inhibited ERK1/2 activation by CypA in VSMC. Importantly, administration of MM284 in vivo significantly inhibited intima formation after carotid artery ligation. Based on these findings, we hypothesize that eCypA is a novel mediator of PAH based on multiple mechanisms: EC apoptosis; VSMC proliferation and migration; and recruitment of inflammatory cells. To test this hypothesis, we propose 3 aims: 1.) Define the mechanisms by which CypA and AcK-CypA mediate PH in ecCypA-Tg mice. 2.) Define the role of eCypA in PH in ecCypA-Tg mice using MM284. 3.) Determine the effect of specific eCypA inhibition on the progression of PH in P-MCT rats. Accomplishing these Aims will provide novel mechanisms and therapeutic strategies for human PAH.